Fatigue strength of 304, cold worked versus annealed

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Discussion Overview

The discussion revolves around the fatigue strength of 304 stainless steel in cold worked versus annealed conditions, particularly in the context of designing components subject to high cycle fatigue. Participants explore the implications of material properties on fatigue performance, including stress concentrations and potential failure mechanisms.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Experimental/applied

Main Points Raised

  • Some participants note that cold working increases the fatigue strength of austenitic stainless steels but reduces it in the presence of notches compared to annealed materials.
  • Participants mention specific endurance limits for 304 stainless steel: 35 ksi for annealed and 92 ksi for 3/4 hard (cold worked).
  • There is a consideration of the design requirements for components expected to exceed 10^6 cycles, with a focus on axial tension/compression and stress concentrations of about 2 or 3.
  • Some participants question under what conditions the quoted statements about fatigue strength are made, noting that cold worked metal has a greater elastic range and yield strength, while annealed material has greater uniform elongation and creep resistance.
  • One participant raises concerns about the lack of information on stress concentration factors for threaded parts, specifically mentioning failures occurring at the thread root after about one million cycles.
  • There is an inquiry into whether a cold worked version of the material would improve fatigue performance in the context of observed failures.

Areas of Agreement / Disagreement

Participants express uncertainty regarding the best material choice between cold worked and annealed stainless steel for fatigue applications, indicating that multiple competing views remain on the implications of material properties and failure mechanisms.

Contextual Notes

Limitations include the lack of comprehensive data on stress concentration factors for threaded parts and the specific conditions under which the fatigue strength statements are made.

Q_Goest
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This web page: http://www.hghouston.com/ss_cwp.html

states:
Cold working increases the fatigue strength of the austenitic stainless steels. However, the fatigue strength of these cold worked alloys is reduced by notches, as compared to notched fatigue strength in the annealed condition.
The web page also shows that fatigue strength can be improved dramatically by cold working. Endurance limit is listed as:
304 annealed = 35 ksi
304 3/4 hard = 92 ksi

I'm in the process of designing something that will be subject to fatigue. Cycles will quickly exceed 10^6 cycles, and even 10^8 cycles will come along all to quickly. The part is in axial tension/compression. It will have stress concentrations of about 2 or 3. But the statement makes me wonder...

Which material would be best, annealed or cold worked? How can this be quantified?
 
Last edited by a moderator:
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Q_Goest said:
This web page: http://www.hghouston.com/ss_cwp.html

states:
Cold working increases the fatigue strength of the austenitic stainless steels. However, the fatigue strength of these cold worked alloys is reduced by notches, as compared to notched fatigue strength in the annealed condition.

The web page also shows that fatigue strength can be improved dramatically by cold working. Endurance limit is listed as:
304 annealed = 35 ksi
304 3/4 hard = 92 ksi

I'm in the process of designing something that will be subject to fatigue. Cycles will quickly exceed 10^6 cycles, and even 10^8 cycles will come along all to quickly. The part is in axial tension/compression. It will have stress concentrations of about 2 or 3. But the statement makes me wonder...

Which material would be best, annealed or cold worked? How can this be quantified?
I'm wondering under what conditions the quoted statement is made. CW metal has a greater elastic range than annealed metal, and obviously higher yield strength.

On the other hand, the uniform elongation of annealed material is much greater, so strain to failure is greater. Annealed materials are more creep resistant than CW material, and perhaps the quote is referring to creep related cracking at the tip of a notch.
Field Metallography can be used as a tool to determine remaining life, fitness for service, and damage assessment from creep mechanisms or fire damage. It is also useful in determining the cause for cracking, degree of overheating or other damage mechanisms manifested by microstructural changes.
http://www.hghouston.com/services_9.html

My question would be - what are the expected stresses as compared to yield strength of ANN and CW steels.
 
Last edited by a moderator:
Hi Astronuc,
Astronuc said:
My question would be - what are the expected stresses as compared to yield strength of ANN and CW steels.
That's difficult to determine because there is no really good information on stress concentration factors for threaded parts. This particular part has a 1.750-8 thread and seems to be failing regularly due to fatigue, generally after about one million cycles. The crack initiates at a thread root.

I'm trying to determine if a cold worked version of the material will improve that or not.
 
Q_Goest said:
Hi Astronuc,

That's difficult to determine because there is no really good information on stress concentration factors for threaded parts. This particular part has a 1.750-8 thread and seems to be failing regularly due to fatigue, generally after about one million cycles. The crack initiates at a thread root.

I'm trying to determine if a cold worked version of the material will improve that or not.
I have an ASTM STP about fracture of threaded fasteners. Let me see if I can find it.
 
Thanks. If you have the spec number I should be able to get it.
 

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